1. Field of the Invention
The present invention relates to an improvement of a pulse generating circuit for generating a steep pulse output by rapidly discharging a charge of a capacitor.
2. Description of the Prior Art
Simple pulse generating circuits for generating a steep pulse output by discharging the charge of a capacitor to feed the discharged current, have been used for generating a spark for a flash for photography or for firing of a gas lighter or a stove.
FIG. 1 is a circuit diagram of one embodiment of the conventional pulse generating circuit wherein the reference numeral 1 designates a DC power source such as a dry battery having 9 to 12V; 2 designates a starting switch; 3 designates a charging resistor; 4 designates a capacitor; 5 designates a trigger element; 6 designates a thyristor; 7 designates a load (load resistor); and 8 and 9 respectively designate output terminals.
The operation of the circuit will now be briefly described.
When the starting switch 2 is turned on, the capacitor 4 is charged from the DC power source 1 through the switch 2 and the charging resistor 3. The charging time constant is given depending upon the resistance of the charging resister 3 and the capacitance of the capacity 4. When the charged voltage of the capacitor 4 reaches to the switching voltage (E.sub.s) of the trigger element 5, the trigger element 5 is turned on to feed the trigger signal to the gate of the thyristor 6 to turn on the thyristor 6. As a result, the charge of the capacitor 4 is discharged through the load resistor 7 and the thyristor 6 to generate a pulse output between the output terminals 8, 9 of both ends of the load resistor 7.
FIG. 2 shows the voltage waveform of the capacitor 4 in such an operation mode. The charging operation is initiated by turning on the starting switch 2. When the charged voltage reaches the switching voltage (E.sub.s) of the trigger element 5 at the time t.sub.1, the charge is immediately discharged to zero. After the discharge, the thyristor 6 is immediately turned off and the charging operation to the capacitor 4 is initiated. The operation is repeated.
Accordingly, as shown in FIG. 2, the charging and discharging operation is repeated in the period t.sub.1 given by the specific switching voltage (E.sub.s) during the time in the ON state of the starting switch 2 to obtain the continuous pulse output between the output terminals 8, 9.
However, in such a circuit, the initiation of the discharge is decided depending upon the specific switching voltage (E.sub.s) given by the trigger element 5.
If the voltage of the DC power source 1 is lowered to be incapable of charging to the switching voltage (E.sub.s), it is impossible to obtain the pulse output because of no discharge. This is a fatal disadvantage.
Accordingly, when the voltage of the DC power source 1 is significantly changed, the switching voltage (E.sub.s) of the trigger element 5 must be set at a relatively low level under the consideration of the change of the voltage of the DC power source. Accordingly, the voltage of the pulse output is lowered whereby the output voltage being proportional to the voltage of the DC power source 1 can not be obtained. When the voltage of the DC power source 1 is decreased to lower than the switching voltage (E.sub.s), the output is not generated. In this case, it is difficult to distinguish this fact from the failure of the circuit to stop the generation of the output.